Back in the Stone Age, humans had to put up with all sorts of creepy crawlies. Parasites — organisms that live on or in another organism — lingered inside our bodies, living off of our blood. Because internal parasites go mostly unnoticed, they were able to keep living with humans. People can survive a long time with tiny parasitic worms in their intestines.

This seems like a win-lose situation — with humans the losers. But scientists now say that may not be so. Parasitic worms might have...